[OpenMP] Codegen support for 'target teams' on the host.

This patch adds support for codegen of 'target teams' on the host.
This combined directive has two captured statements, one for the
'teams' region, and the other for the 'parallel'.

This target teams region is offloaded using the __tgt_target_teams()
call. The patch sets the number of teams as an argument to
this call.

Reviewers: ABataev
Differential Revision: https://reviews.llvm.org/D29084

llvm-svn: 293001